1. Technical Field
The present invention relates to rotors for electric rotating machines that are used in, for example, motor vehicles as electric motors and electric generators.
2. Description of Related Art
Japanese Patent Application Publication No. 2006-353041 discloses a rotor for an electric rotating machine. The rotor includes a rotor core, a plurality of permanent magnets, and a pair of annular end plates. The rotor core is formed by laminating a plurality of annular magnetic steel sheets in the axial direction of the rotor core. The permanent magnets are embedded in the rotor core so as to form a plurality of magnetic poles which are spaced in the circumferential direction of the rotor core at predetermined intervals and the polarities of which alternate between north and south in the circumferential direction. The end plates are respectively fixed on a pair of axial end faces of the rotor core.
With the above configuration, however, if the end plates are omitted from the rotor for cost reduction, during rotation of the rotor, the magnetic steel sheets may become separated from one another in the vicinities of radially outer peripheries of the axial end faces of the rotor core.
The above problem may be solved by welding the magnetic steel sheets as disclosed, for example, in Japanese Patent Application Publication No. H6-86487. More specifically, the magnetic steel sheets may be welded to form, on the radially outer surface of the rotor core, a plurality of welds each of which extends in the axial direction of the rotor core.
However, with the above formation of the welds, during operation of the electric rotating machine, for each of the welds, all parts of the weld will experience the same change in magnetic fluxes perpendicular to the axial direction of the rotor core at the same time. Consequently, a large eddy current will be induced in each of the welds, thereby considerably lowering the efficiency of the electric rotating machine.